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The Turtle Island (Kueishantao) is situated in the southernmost Okinawa Trough, a back-arc basin to the north of the Ryukyu Arc. Based on bathymetry around the Turtle Island, obviously hummocky seafloor indicating slope failures have occurred. The Turtle Island could have erupted four times in the last 7000 years. Previous studies have pointed out that volcanic eruptions may be related to island flank collapses. The bathymetric hummocky feature reveals submarine debris avalanches in the north, south and east side. In order to understand the landslide mechanism, we have conducted detailed marine geophysical surveys in the north part of the debris avalanches. The multi-beam bathymetric data, sub-bottom profiler, sidescan sonar, sparker seismic reflection data and remotely operated vehicle (ROV) dives data are used to identify the landslide features, including the depositional and erosional characters. Our results show that the debris avalanche is probably related to the horseshoe scar of the subaerial flank collapsed event. The debris avalanches display a runout distance of up to 4 km northeastward and form a ~4 km2 hummocky terrain with displaced blocks. The biggest block could be ~50 m high and 50 m in diameter. Identified by the sparker seismic data, the chaotic facies of the landslide deposits can be divided into three MTD (Mass Transport Deposit) units (MTD1, MTD2 and MTD3). The main volcanic debris avalanche deposits are identified as MTD3 with a characteristic of broken and discontinuous lateral reflections. Correlated with seismic data and sub-bottom profiles, multiple landslide events are recognized on the basis of different MTD units. The last landslide event is probably related to a faulting and a slope failure of the island and could be highly associated with a climatic event.

Okinawa Trough (OT) is a back-arc basin behind the Ryukyu arc-trench system and located along the eastern margin of the Eurasian continent. It extends from the southwest Kyushu Island to the Ilan Plain of Taiwan (Fig. 1). The width and water depth vary from about 230 km and 200 m in its north to 60 - 100 km and 2300 m in its south (Sibuet et al. 1998). Based on geophysical and geological studies, the Okinawa Trough is considered to be in the rifting stage (Lee et al. 1980; Letouzey and Kimura 1986; Sibuet et al. 1987, 1998; Uyeda 1987; Kimura et al. 1988). Such a tectonic setting is characterized by active normal faulting in its upper crust and numerous intrusions of magma. Hence, the OT is expected to have a favorable environment for the development of a hydrothermal system. Recently, diverse microbial communities and other hydrothermal minerals have been discovered in the middle and northern Okinawa Trough (Uyeda 1987; Kimura et al. 1988; Halbach et al. 1989; Sakai et al. 1990; Glasby and Notsu 2003; Lüders and Niedermann 2010; Ishibashi et al. 2014; Toki et al. 2016; Minami and Ohara 2017). The southern OT is close to Taiwan and topographically has the lowermost trough throughout the Okinawa Trough (Fig. 1), suggesting a pronounced back-arc basin. However, restricted by the limited sampling and sequencing studies, our understanding of its marine environment and hydrothermal potential is poor. Supported by Central Geological Survey (CGS), Ministry of Economic Affairs (MOEA) of Taiwan, a 4-year program entitled “Geological Investigation of Mineral Resource Potential in the Offshore Area of Northeastern Taiwan” was launched in 2016. Significant contributions have been made in understanding the origin, distribution, and characteristics of hydrothermal deposits in the southern Okinawa Trough. Integration of geological, geophysical, geochemical and seafloor observation studies of the background environment is very helpful for understanding the hydrothermal mineral deposits in the southern OT.

This paper aims to understand the hydrothermal sites near the Yonaguni Knoll IV in the Okinawa Trough, and to develop new techniques to study fluid flow patterns for hydrothermal systems and their impact on ore deposits on the seafloor. Hydraulic parameters are important for hydrothermal system studies, but in-situ measurements of fluid migration rates are difficult. Hydrothermal fluids can reach several hundred degrees Celsius, temperatures high enough to perturb hydrothermal fields and pore water migration patterns. Using in-situ temperature data as constraints, we model and synthesize 1-D and 3-D cylindrical hydrothermal models to fit the spatial variations of observed temperature fields. The 1-D modeling uses Péclet number analysis along the conduit. We also construct a 3-D cylindrical model to estimate the temperature and fluid velocity fields using a finite element software. All domains are set to be porous to allow the fluid to flow. The simulation is run until it reaches a semi steadystate solution, allowing both the temperature and velocity fields to stabilize. Results show the dimension of the thermal anomaly zone is likely controlled by advective heat transfer along the vent due to upward fluid flow. We estimate a Péclet number of -1.6, and the vertical fluid flow velocities at these sites are high, approximately 106 m s-1, that is, about 100 m yr-1. This is a spatially averaged estimate over tens to hundreds of meters and does not take into account finer-scale venting, which may be very heterogeneous. The results of this work may help estimate the quantity of metal elements transported through pore fluid migration at modern hydrothermal sites.

Using echo sounders to detect gas plumes in seawater is common, especially in the context of hydrothermal circulation areas or gas hydrate-bearing cold seeps. To understand the distribution of gas plumes in the southernmost Okinawa Trough, we have conducted 13 cruises with a 38 kHz single-beam echo sounder (EK60). A total of 266 gas plumes of acoustic image, associated with the hydrothermal circulation, are detected. To estimate the near-seafloor bottom current speeds, 201 gas plumes are further used. As a result, the gas plumes around the axial depression of the Okinawa Trough generally tilt to the northeast at rising tides and high tides, suggesting a northeastward flow of the bottom current. However, the gas plumes in the Keelung continental slope tilt to the southwest at ebb tides and low tides, suggesting a southwestward flow of the bottom current. Our results significantly show a good estimation of the near-seafloor bottom currents from EK plume images in the case of lacking real observations. The directions of the bottom currents depend on semidiurnal tides. Assuming a quasi-constant speed of upward gas bubbles out of seabed, we have estimated the bottom current speeds in 6 hydrothermal circulation zones near the rifting center of the southernmost Okinawa Trough. The estimated bottom current speeds in submarine volcanic areas vary largely from 2 to 160 cm s-1, but bottom current speeds in relatively flat region are between 20 and 50 cm s-1. The large variation of the bottom current speeds in the submarine volcanic areas could be due to the variable emissions of the gases out of the submarine volcanic areas.

To explore a new discovering hydrothermal field Geolin Mounds in the Southern Okinawa Trough (SOT), a multi-scale geophysical investigation cruise including single-beam echo sounder, sub-bottom profiler (SBP) and multi-channel seismic (MCS) surveys were conducted in 2018 in a new discovering hydrothermal field we named Geolin Mounds. Taking advantage of streamer feathering caused by strong Kuroshio Current during MCS data acquisition, we performed a pseudo-3D processing and produced a pseudo-3D seismic cube. In addition to flare features in water column, "rock grove" like morphological feature above seafloor, widely-distributed amplitude anomalies including blanking zone and high-amplitude reflectors are observed around the Geolin Mounds hydrothermal field in our 2D MCS data. Pseudo- 3D seismic cube, on the other hand, provide estimation of the areas of blanking zone on selected time slice; furthermore, better characterize the fault structures in the hydrothermal field. The Geolin Mounds is the first disclosed site in the SOT where a hydrothermal field is without underlying submarine volcanos. We thus suggest that the Geolin Mounds hydrothermal field is in its embryo stage of evolution and is constantly supported by vigorous subsurface hydrothermal circulation. Consequently, the Geolin Mounds hydrothermal field may grow sustainably and serves as a good observatory for development of the seafloor edifice and ore mineralization associated with hydrothermal circulation activities.

With large topographic gradients, rifted basins serve as efficient traps for particulate matter from adjacent lands and the ocean surface. However, the fate of organic carbon in the sediment, mostly unaltered by the hydrothermal activities known to occur in young rifts, remains poorly understood. In this study, we present an examination of diagenetic activities and carbon remineralization based on the first complete suite of pore-water data of sediment marginally affected by hydrothermal activities in the Southern Okinawa Trough (SOT). The sediment showed an oxygen penetration depth of 1 cm, consumption of NO3- in the top 1 cm, smeared profiles of Mn2+ and Fe2+ with the latter reaching up to 450 μmol L-1, and relatively unchanged SO42- concentrations with depth. Net production rates of dissolved species resolved from pore-water profiles provide an estimate of 1.68 ± 0.21 mmol C m-2 d-1 as the total carbon remineralization rate in the upper 30 cm sediment column, with aerobic carbon oxidation being the major pathway. The rate, one order of magnitude lower than that of the adjacent East China Sea, is attributed to the lower bottom-water temperature and carbon flux in the trough. The high carbon burial efficiency of SOT (68% of carbon reaching the seafloor and processed thereunder) reflects the combined effects of small mountain rivers and rifting-induced particle trapping.

Multi-scale geophysical survey including pseudo-3D seismic, sub-bottom profiling, side-scan sonar, multi-beam and single-beam bathymetry, heat flow investigations as well as bottom-water, core and dredge sampling works have been conducted in a newly discovered hydrothermal field named as Geolin Mounds at about 1,510 depth in the Okinawa Trough. Ship-mounted bathymetric data cannot detect specific morphological features on the seafloor in this field; however, “rock grove” morphological characteristics are observed by using deep-towed side-scan sonar. Moreover, vigorous flare features in water column are detected by multi-beam and single-beam echo sounder, and widely distributed high heat flow anomalies (> 10,000 mW/m2) also exist in the survey area. Due to strong Kuroshio Current during our multi-channel seismic survey, 5-40°streamer feathering effect occurred. To take advantage of swath distributed seismic reflection points caused by streamer feathering, we developed a pseudo-3D technique and produced a 3D seismic cube from this uneven seismic dataset. The 3D seismic imaging and sub-bottom profiling results indicate widely-distributed anomalies such as blanking zone and high-amplitude reflectors around the Geolin Mounds hydrothermal field and could link the specific features above seafloor. The 3D seismic cube also provides better estimation of the areas of blanking zone on selected time slice and better characterizes fault structures in the hydrothermal field. The geochemical analysis results present high Ag, Au, As, Bi, Cd, Cu, Fe, Pb, Sb, and Zn concentrations have been found in our coring and dredging samples. Relatively high concentration of methane, rare earth elements and 3He/4He ratio in near bottom seawater samples are also shown. Based on the geophysical and geochemical works, we propose that the Geolin Mounds hydrothermal field is without underlying submarine volcanos and hydrothermal fluid migration could be related to fault development. This hydrothermal field is in its embryo stage of evolution and constantly supported by active hydrothermal circulation. As a consequence, seafloor massive sulfides deposits and related geomorphological features can keep developing with hydrothermal fluid circulation along migration conduits. The fault structures and volcanic activities due to back-arc spreading process in the Southern Okinawa Trough should dominated creatures of those fluid migrating conduits. We suggest that the Geolin Mounds hydrothermal field could sustainably grow and have high potential of massive sulfides resources in the Southern Okinawa Trough. Furthermore, this hydrothermal field can serve as a good observatory for get better understanding of seafloor edifice development and ore mineralization associated with hydrothermal circulation activities in a back-arc spreading tectonics.

The southernmost Okinawa Trough back-arc basin is an active and young basin formed just after the collision of the Philippine Sea Plate against the Eurasian continental margin. The back-arc extension occurs intensively because of the southward or southeastward migration of the southernmost Ryukyu Arc, or the roll-back of the Philippine Sea Plate. To better understand the active tectonics and volcanism of the southernmost Okinawa Trough, we have conducted deep-tow sub-bottom profiler and side-scan sonar surveys across the back-arc basin. Our results show that the volcanism of the southernmost Okinawa Trough is distributed in the southern half of the back-arc basin and occurs along some linear or branched zones roughly parallel to the trough axis. Volcanic seamounts are obviously located along the central depression of the basin and their sizes show lateral variation. On the other hand, the northern half of the southernmost Okinawa Trough back-arc basin has almost no volcanic activity, but contains more brittle normal faults. It is noted that gas plumes out of seafloor are generally associated with hydrothermal mounds or activities, instead of volcanic seamounts. We suggest that the more complete rifting of the southernmost Okinawa Trough back-arc is limited to the east of ~122o30′E. To the west of ~122o30’E, the back-arc extension could be still influenced by the inherited NE-SW trending structures of the continental crust created during the former Taiwan orogeny in this area.

Voluminous hydrothermal systems are corresponding to the heat flow anomaly discovered from previous geophysical surveys in the Southern Okinawa Trough (SOT), which is an area with a high sedimentation rate supplied by the East China Sea shelf and the island of Taiwan. Our latest fifteen field measurements around the Geolin Mound (GLM) (24° 36.6 ‘N, 122° 53’E) are located at a sedimentary basin west of the Yonaguni Rift, which is nearby the southwestern tip of a NE-SW trending volcano trail in the SOT. The heat flow results illustrate a concentric decrease from 31,477 to 180 mW/m2 in a 1-km radius around the GLM, evidently proving the focus fluid migration along the major fluid conduit. The ultrahigh temperature observations revealed the existence of magmatic components in shallow sediments. Combining the subsurface profiles with the temperature fields around the GLM, a 0.16-km2 acoustic blanking area of the G1 fluid conduit was recognized from the chirp sonar image and numerous interpreted igneous reflections with fluids related features from seismic profiles. As a result, we proposed a conceptual model, which simultaneously illustrates the hydrothermal circulation pattern from the previous general surveys and the enormous thermal anomaly from current detailed measurements. The sill-to-sill feeding sedimentary basin of the GLM is capable of demonstrating the enormous thermal anomaly around the spreading center of the SOT and appropriately represents the potential hydrothermal discharge and recharge by heat flow distribution around the GLM.

This paper presents the seafloor survey results obtained in 2017 using two deep-towed vehicles, FITS and TVG, developed by the Institute of Undersea Technology at National Sun Yat-sen University, for investigating mineral resource potential offshore northeastern Taiwan. The seafloor surveys were conducted at five sites in the Southern Okinawa Trough via three cruises. A total of 39.4 kilometers of seafloor observation was accomplished, and plenty of seafloor characteristics and ecological phenomena were observed through FITS and TVG in the three cruises.

Yonaguni Knoll IV is an active hydrothermal venting site located on the southern tip of a cross back-arc volcanic trail in the South Okinawa trough. We have compiled 22 heat-flow measurements on the seafloor around the Yonaguni Knoll IV site that reveal drastic heat-flow spatial variation. To obtain the appropriate heat-flow measurements from our recently rebuilt Lister-type marine heat probe, we have updated the processing algorithm in this study to avoid utilizing the sometimes-unstable temperature evolving data from the frictional heating stage. We have carefully calibrated and processed the observed data to yield robust estimates of the seafloor heat-flow emphasizing in particular that the previous implementation of the data reduction scheme by downsizing model parameters into two subsets to be pursued in two separate steps is not theoretically and practically essential. We also examine the trade-offs of sensitivity on model parameters constrained by the thermal decay data and reveal previously unclear knowledge concerning the intertwined mutual dependence that is critical to the performance the data reduction. A contrast of more than 3000 mW/m2 within short distances implies active local hydrothermal circulation. The observed low heat-flow area could be a cold seawater-charging site whereas Yonaguni Knoll IV might stand for a close by discharging output. Although it demands for detailed mapping to establish the 3D geometry of the local hydrothermal circulation, preliminary evidence suggests that the lateral dimension of the circulation cell is around a few kilometres.